CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and any other benefit of, U.S. Provisional Patent Application Serial No. 63/263,923 entitled LIGHT PROTECTION SYSTEM AND METHOD, filed November 11, 2021, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

Embodiments of the subject matter disclosed herein relate to a system and method of using optical polarization techniques to attenuate light from a light source.

BACKGROUND

In many situations, it is important to control impinging undesirable light to prevent harm or damage to, for example, a user’s eyes, devices, areas, sensitive equipment, among others or to allow better visibility or contrast for systems such as augmented or mixed realities. Eye damage due to laser light or spark flash may be of great concern. For example, laser light can cause eye damage to pilots or first responders, and spark flash can cause eye injury to electrical grid works.

To accommodate these concerns, electronically switchable optical filters (active optical filters) may be used to attenuate light in various applications to adjust/reduce the level of light passing through the device. By way of example and not limitation, attenuating light filters can be used in goggles, glasses, windshields, and windows, among others.

Various devices use optical filters to attenuate and adjust levels of light passing through them. One method of light attenuation is through an optical filter formed from a pair of polarizers that sandwich a conventional liquid crystal waveplate that acts as an electronically controllable polarization rotator. This geometry is commonly used in conventional LCDs. In these filters the maximum optical attenuation level is dictated by the performance of the polarizers. As an example of such a device, FIG. 1 illustrates an optical filter 100 formed from a first static (not electronically switchable) absorptive polarizer 102 and a second static absorptive polarizer 104 that acts on light 120 from a light source to produce transmitted light 130 which may be attenuated in intensity. The first static absorptive polarizer 102 is configured to polarize light on a first axis (horizontal) and the static absorptive second polarizer 104 is configured to polarize light on a second axis orthogonal to the first axis (vertical), thereby forming a pair of crossed polarizers. Crossed polarizers are highly effective at eliminating light incident normal to its surface (i.e., normally incident light). However, they do not operate as effectively when light is incident at oblique angles. Therefore, the attenuated intensity from a wide viewing angle has a cross pattern. FIG. 2 illustrates a typical cross pattern 200 of attenuated light formed by a pair of broadband cross polarizers of FIG. 1. This is an inherent limitation of conventional polarizerbased devices and results in light that is attenuated less effectively at the outer “corners”. The light leakage at the outer corners results in an inhomogeneous attenuation of light that limits the optical performance of the filter. The role of the liquid crystal wave plate in these devices is to control the polarization state of the light that has exited the first polarizer without absorptive losses. Its presence, however, results in decreased field of effective attenuation (FEA). As such, a significant amount of effort is placed in finding ways to mitigate this negative effect, including using compensated polarizers, etc.

For example, FIG. 3 illustrates an approach to address this reduced FEA. Optical filter 300 includes two horizontal polarizers and two vertical polarizers, where the two vertical polarizers are in the center. In other words, the optical filter 300 is formed from two sets of crossed static absorptive polarizers, namely a first polarizer pair 306 and a second polarizer pair 312 that acts on light emerging from 306 to produce transmitted light 330, which may be attenuated in intensity. The first and second polarizer pair 306, 312 each include a horizontal polarizer 302, 308 and a vertical polarizer 304, 310, respectively. Fig. 4A shows the pattern of light attenuation formed by the prior art optical filter 300 of FIG. 3.

The FEA can be further improved by rotating the second set of cross polarizers with respect the first. Fig. 4B shows the pattern when the second cross polarizer pair 312 is rotated at a 45-degree angle to the first pair 306. This configuration can improve device inhomogeneity of attenuated light but suffers from other limitations, i.e., reduced clear state transmission, which is undesired in certain applications.

In the optical filter examples above, a sandwiched liquid crystal cell can be used to alter the polarization of light emerging from the first static absorptive polarizer and thus alter light transmission through the second static absorptive polarizer. For example, FIG. 5 illustrates a prior art example using an active liquid crystal waveplate (or polarization rotator) in optical filter 500 that includes a first static absorptive polarizer 502, a second static absorptive polarizer 504, and a conventional liquid crystal cell 510 located between the first and the second polarizers 502 and 504. The polarizers 502 and 504 are plane polarizers where the first polarizer 502 is rotated 90 degrees from the second polarizer 504. Optical filter 500 acts on light 520 from a light source to produce transmitted light 530. The conventional liquid crystal cell 510 is a non-absorbing, electrically controllable phase retarder or waveplate, that can rotate the polarization of light by 90 degrees. By alternating the voltage applied to the waveplate 510, optical filter 500 can alternate between a clear and a dark state.

Similarly, two or more active liquid crystal waveplates can be used in more complex configurations. FIG. 6 illustrates a prior art optical filter 600 that includes a plurality of liquid crystal layers (608, 610, 612) and a plurality of static absorptive polarizers (602, 604, 606). The polarizer 602 is rotated ninety-degrees from the direction of the polarizers 604, 606, which are parallel to each other. Optical filter 600 acts on light 620 to produce transmitted light 630, which may be attenuated in intensity across a broad spectrum. When activated, the active liquid crystal cells can rotate the polarization of received light to achieve a desired darkness. For wide-band applications, this arrangement is complex and not ideal for near-eye applications due to its low clear state transmission.

For near eye applications, the need for a wide visible angle plays a significant role in the applicability of the device. This has hindered the implementation of electronically variable shutters in a variety of eyewear form factors. Therefore, it is desirable to provide simple optical filters that have good visible angle dependence for near-eye applications.

SUMMARY

An optical filter for attenuating a light source is provided. The optical filter includes first and second outer polarizers each configured to effectively polarize light in a first polarization axis. A first inner switchable polarizer is disposed between the first outer polarizer and the second outer polarizer. The first inner switchable polarizer is configured to alternate between a more polarizing state and a less polarizing state upon a change of an applied voltage. The first inner switchable polarizer configured to polarize light on a second polarization axis effectively orthogonal to the first polarization axis when the first inner switchable polarizer is in the more polarizing state. The change in applied voltage alters a transparency of the optical filter between a more transparent state and a less transparent state. The switchable polarizer may include a liquid crystal cell. The optical filter may include multiple switchable polarizers.

These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the accompanying drawings in which particular embodiments and further benefits of the provided subject matter are illustrated as described in more detail in the description below.

FIG. 1 shows a prior art optical filter formed from a pair of crossed static absorptive polarizers.

FIG. 2 shows a typical pattern of light attenuation by the optical filter of FIG. 1.

FIG. 3 shows a prior art optical filter formed from two horizontal polarizers and two vertical polarizers, where the two vertical polarizers are in the center.

FIG. 4A shows a typical pattern of light attenuation by the optical filter of FIG. 3.

FIG. 4B shows a typical pattern of light attenuation by the optical filter of FIG. 3 where the second pair of polarizers are rotated at a 45-degree angle from the first pair.

FIG. 5 shows a prior art optical filter formed from a pair of static absorptive polarizers and a single non-absorbing polarization rotator.

FIG. 6 shows a prior art optical filter formed from multiple static absorptive polarizers and multiple non-absorbing polarization rotators.

FIG. 7A is a non-limiting depiction of a Field of Effective Attenuation (FEA).

FIG. 7B is a non-limiting aperture that may be acceptable for an optical filter.

FIG. 7C is another non-limiting depiction of a FEA for a human.

FIG. 8 is a schematic of a non-limiting example of an optical filter system according to some embodiments.

FIG. 9 is a cross-sectional schematic view of a non-limiting example of an optical filter system in a transmissive state according to some embodiments.

FIG. 10 is a cross-sectional schematic view of a non-limiting example of an optical filter system in a darkened state according to some embodiments.

FIG. 11 is a cross-sectional schematic view of a non-limiting example of a guest host liquid cell switchable polarizer according to some embodiments.

FIG. 12 is a cross-sectional schematic view of a non-limiting example of an optical filter system according to some embodiments. FIG. 13 is a representation of the absorbance of light when a red filter of the optical filter of FIG. 12 is activated

FIG. 14 is a representation of the absorbance of light when a red filter, a blue filter, and a green filter of the optical filter of FIG. 12 are activated.

FIG. 15 is a cross-sectional schematic view of a non-limiting example of an optical filter system according to some embodiments.

DETAILED DESCRIPTION

DEFINITIONS

Unless specifically defined otherwise herein, the definitions for optical parameters such as linear, circular, and unpolarized light are the same as those in "Principles of Optics Electromagnetic Theory of Propagation, Interference and Diffraction of Light", Max Born, et al., Cambridge University Press; 7th edition (Oct. 13, 1999). Similarly, all liquid crystal terminology which is not specifically defined herein is to have the definition as used in Liquid Crystals Applications and Uses, vol. 3, edited by B. Bahadur, published by World Scientific Publishing Co. Pte. Ltd., 1992 ("Bahadur").

“Acceptance Aperture” refers to the total angle where the optical density is above an accepted value.

“Active” as used herein means electronically switchable.

“Band Pass Filter” as used herein, is defined as a device that allows light of a certain range of wavelength(s) to pass through but rejects the passage of light of other wavelengths.

“Clear” or “Clear State” means a maximally transmissive state. In some embodiments, a clear state has a light transmission greater than 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 85%, or 90%.

“Contrast Ratio” as used herein, is defined as a ratio of the transmitted light in fully transmissive state to the transmitted light in the fully dark state. In some embodiments, the contrast ratio is greater than 2, 5, 10, 25, 50, 75, or 100.

“Dark” or “Dark State” means a minimally transmissive state. In some embodiments, a dark state has a light transmission of 25%, 10%, 5%, 1%, 0.5%, 0.1%, 0.01%, 0.001%, 0.0001% or less. “Effective Polarization” or “Effective Polarization Axis” refers to the polarization or polarization axis produced by a polarizer (linear, circular, or other) either alone or in combination with an adjacent waveplate if one is present.

“Field of Effective Attenuation” or FEA is a solid angle beyond which the optical density drops by a preset value. The preset value can be greater than 0.1, 0.2, 0.3, 0.5, 1, 1.5 or 2 ODs.

“Narrow-Band” region of the spectrum is a region having a full-width, half-max of less than 175 nm.

“Optical device” is any device that can control the transmission of light. For example, a viewer or sensor can see through the device.

“Optical filter” as used herein, is a device that selectively transmits light of different wavelengths or different wavelength regions. A “wavelength region” refers to a band within the electromagnetic spectrum with a range of wavelengths, the band may be narrow (<175 nm) or wide (<175 nm).

“Orthogonal Axis” refers to the orientation of a circular or linear polarizer element axis with respect to an incident polarized light such that a least transmission of the circularly or linearly polarized light respectively is achieved.

“Optical Density” or “OD”: OD is related to the absorbance (A) and is defined as:

OD = - log A

Transmissivity, T, is then related to the OD as:

T=10' ^OD

“Static” means a non-switchable or passive layer.

“Polarizer” refers to a material that absorbs or reflects one polarization of incident light more than the orthogonal polarization.

“Polarization Axis” refers to the direction along which a polarizer passes the electric field of an electromagnetic wave. For example, an absorptive or reflective polarizer having an X polarization axis will preferentially transmit the x-direction polarization and absorb or reflect a portion of the y-direction polarization of incident light, etc. “Switchable Polarizer” refers to an optical layer that can be switched or altered, by application of voltage change, to alternate between a more polarizing state (absorbing or reflective) and a less polarizing (more transmissive) state. In other words, in the more polarizing state, the polarizer preferentially absorbs or reflects one polarization of light at a greater rate than in the less-polarizing state. In a less polarizing state, the polarizer will effectively transmit both polarization axes of light. The polarization level of the switchable polarizer is selected by controlling the voltage applied. Depending on the application, a switchable polarizer can be in a maximal polarizing state when a max voltage is applied, or vice versa, (i.e., be maximally polarizing when the voltage is 0).

“Tinted” state as used herein, is defined as a state between the maximally transmissive (clear) and the minimally transmissive (dark) states.

“Transparent” is used interchangeably with Clear.

“Wide-band” or “Broad-band” region of the spectrum is a region having a full-width, half-max equal to or greater than 175 nm.

As used in this application, the terms “x” and “y” polarization axes are arbitrary and refer to a first and second polarization axis of light which are orthogonal axes to each other and can be linear, circular, or elliptical. For example, they may be used to describe a first and second polarization axis. This is used to simplify and/or aid in the disclosure of the present application and is not to be construed as any fixed values or axis. Furthermore, terms such as parallel and perpendicular may be used to describe two orthogonal polarization states, which can be circular, elliptical, or linear, for example. In some cases, a first polarization axis may refer to a right- handed circularly polarized light, and an orthogonal second polarization axis may refer to the opposite left-handed circularly polarized light (or vice versa with respect to right- and lefthandedness). These terms, parallel and perpendicular, can also be used in the same sense to describe the response of the polarizers to the incident polarization state.

Unless otherwise noted, the polarization created by a polarizer generally refers to its “effective” polarization in the context of the optical device and adjacent optical layers such as waveplates (as a non-limiting example). Similarly, an axis of polarization that is described as parallel means “effectively parallel” in the context of the optical device, and an axis of polarization that is described as orthogonal means “effectively orthogonal” in the context of the optical device and adjacent optical layers. Although the term “effective” may not always be used out of brevity, it is generally implied unless otherwise specifically noted or inconsistent with the discussion For example, it is well known that the polarization state of the light entering or exiting a polarizer can be altered with appropriate use of waveplates such as a half-wave plate, quarter-wave plate etc. In such a case the polarizer is creating an “effective” polarization. This effective polarization can be linear or circular and can have an axis which matches or is different from the axis of the polarizer. For example, transmitted light from a circular polarizer can be turned into a linear polarization by use of a proper quarter-wave plate. Conversely, transmitted light from a linear polarizer can be turned into a circular polarization by use of an appropriate quarter-wave plate placed at the appropriate angle with respect to the polarizer axis. This allows for other polarizer systems or configurations to be used. For example, a pair of perpendicularly oriented static polarizers can be utilized as outside polarizers in conjunction with appropriate static wave-plates instead of two parallel-oriented polarizers in the examples described below. Similarly, a combination of circular and linear polarizers with appropriate wave-plates can be used instead of two reflective or two linear polarizers to match the operation of a switchable polarizer. For example, an absorptive switchable polarizer can be used in conjunction with two circular polarizers if additional wave plates are used.

With reference to the drawings, like reference numerals designate identical or corresponding parts throughout the several views. However, the inclusion of like elements in different views does not mean a given embodiment necessarily includes such elements or that all embodiments of the invention include such elements.

The systems and methods described herein provide optical fdters that can increase peripheral performance, response time, or the contrast ratio of the overall device. For example, by utilizing three polarizers in a configuration such that the two outer polarizers are static absorptive or reflective polarizers with a polarization axis effectively parallel to each other, and the middle or inner polarizer is an electronically switchable polarizer configured to be orthogonal to the other two. In such a configuration, the discernable Field of Effective Attenuation or FEA can be controlled or modified. Therefore, this arrangement may be used to enhance the angular dependence of a polarization-based shutter and result in an increased FEA.

In an exemplary embodiment, an optical filter can include a first static absorptive or reflective polarizer, a second static absorptive or reflective polarizer having a polarization axis parallel to the polarization of the first polarizer, and a switchable polarizer provided between the first polarizer and the second polarizer. The first and second polarizers may sometimes be referred to herein as outer polarizers and the switchable polarizer in the middle may be referred to as an inner switchable polarizer. The switchable polarizer may include a switchable liquid crystal (LC) cell having a polarization direction that is alterable by an application of (or removal of) voltage to the LC cell, i.e., applying a voltage change. Therefore, by applying a voltage change to the LC cell, the transparency of the optical filter can be adjusted between a clear state, a dark state, or any state therebetween, where the optical filter absorbs or reflects more light in the dark state than in the clear state. In some embodiments, the electronically switchable polarizer utilizes an LC cell configured to reflect light within a broad-band or a narrow-band region of the electromagnetic spectrum. In some examples, the electronically switchable polarizer utilizes a guest host LC configuration. In some embodiments, the first and/or second polarizers may instead be electronically switchable outer polarizers having a polarization axis substantially orthogonal to that of the inner switchable polarizer.

It should be noted that switchable polarizer LC cells of the present disclosure are different than conventional LC cells of the prior art discussed in the background. As mentioned, conventional LC cells such as those of FIG. 6 include waveplates designed to rotate the polarization of received light without reducing the transmitted energy. In other words, they are ideally designed to transmit all of the incident light but rotate its polarization. Conversely, switchable polarizer LC filters of the present disclosure are configured to reduce (e.g., by absorption or reflection) the incident light transmitted energy.

In another embodiment, an optical filter is configured to selectively reduce narrow-band regions of the spectrum (for example for laser eye-protection or similar applications). This configuration can include a first outer polarizer, a second outer polarizer having a polarization direction the same as the first polarizer (in parallel), and two or more electronically switchable polarizers (inner polarizers) located between the first and the second outer polarizers. For example, the two or more switchable polarizers can include liquid crystal cells configured to polarize in a direction effectively orthogonal to the first and the second outer polarizers. The inner switchable polarizers can be individually controlled and configured such that each, or at least one, operates (polarizes) a narrow-band region of the spectrum. The optical filter can then selectively transmit or block a narrow-band region of the spectrum as needed. This feature can have applications to block intense light such as from a laser without blocking other regions of the spectrum.

In some embodiments, the optical filter may include three switchable polarizers. The first switchable polarizer (first liquid crystal cell) can be placed between the first outer polarizer and the second switchable polarizer (second liquid crystal cell). The third switchable polarizer (liquid crystal cell can be placed between the second switchable polarizer (second liquid crystal cell) and the second outer polarizer. For instance, the order of components can be: the first outer polarizer, the first switchable polarizer, the second switchable polarizer, the third switchable polarizer, and the second outer polarizer.

The arrangement of components described herein will affect the size and quality of the Field of Effective Attenuation or FEA. FIG. 7A illustrates an exemplary FEA 700 for a human being. In this example, the line 702 represents a normal direction of zero degrees. Angle a represents the angle from normal 702 that can define an acceptance aperture. The acceptance aperture can be related to angle a such that angle a represents the angle in which the optical density (OD) rating drops by a percentage, e g., one (1) compared to the OD rating at normal 702. For example, a device that has an OD3 rating at normal 702 may have an OD2 rating at angle a. In an example, the acceptance aperture or FEA may be 30 degrees from normal 702, illustrated by the line 708. For instance, location 704 illustrated by line B-B represents a 60- degree FEA with an OD3 rating. The location 706 illustrated by line B-C represents a location having an OD2 rating. It should be appreciated that any angle may be used to define an acceptance aperture and that 30 degrees is provided solely for exemplary purposes. The FEA value may be 5°, 10°, 15°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, 90°, 120° or higher.

Further, there may be a gradient in optical density and the dashed line 708 may simply indicate where one side is closer to OD = 3 and the other side is closer to OD = 2.

FIG. 7B illustrates an exemplary acceptance aperture 750 when looking through a polarizer. The acceptance aperture is defined in the middle portion 752 of the aperture which may have an OD3 rating. The corners or outer portions 754 can be considered outside of the acceptance aperture and may have a lower OD rating, e g., OD2. In an example, having a large acceptance aperture 752 is desirable over a smaller acceptance aperture because a larger acceptance aperture can lead to a larger FEA. The size of the acceptance aperture can be controlled to mitigate undesirable light leakage at high angles as illustrated by cross pattern 200, for example. A device that is more efficient at blocking out intense light sources can have a higher acceptance aperture and, therefore, may offer a wider FEA.

FIG. 7C illustrates an example of a field of view 770 for a human being, measured in degrees eccentric to fixation, in which the human eye can perceive images and detect motion. In certain applications, it is desirable to make the FEA such that it matches the field of view 770. In this example, the field of view or FEA is about 155 degrees for each eye, or a total 190 degrees for both eyes together. In other examples, to achieve a desirable OD rating, a device can be configured to produce a reduced FEA by presenting a smaller viewing window to account for the configuration of the polarizers. Therefore, it may be desirable to produce an optical filter that can achieve an adequate OD rating while maintaining a wide FEA.

FIG. 8 illustrates a system 800 configured to attenuate light from a light source. The optical device 802 can include an optical filter 804, a sensor 808, a controller 810 including a processor and memory, and a power source 812. The system 800 can include one optical device 802 or a plurality of optical devices. The optical device 802 can include one optical filter 804 or a plurality of optical filters. Similarly, sensor 808 may include a single sensor or represent a plurality of sensors. It should be appreciated that the system 800 can be configured with and can include all of or a portion of the components mentioned elsewhere herein. Similarly, the system 800 can optionally include additional components not mentioned herein without departing from the scope of the invention.

In some embodiments, the optical device 802 can include one or more sensors 808 for sensing an amount of light energy from the light source 820. In an example, the sensor 808 can be chosen from any of the following types: photoresistors, photodiodes, phototransistors, and the like.

The controller 810 can include one or more processor(s) configured to include computerexecutable instructions such as instructions such as a control process for an optical filter device. Such computer-executable instructions can be stored on one or more computer-readable media including non-transitory, computer-readable storage media such as memory. For instance, memory can include non-volatile storage to persistently store instructions, settings, configuration settings, parameter settings, light intensity setpoints, and/or data. Memory can also include volatile storage that stores instructions, other data (working data or variables), or portions thereof during execution by processor. The power source 812 can include a battery of any of the following technologies: lithium ion, lithium polymer, nickel cadmium, nickel-metal hydride, lead acid, or any other suitable composition. In other examples, the power source 812 can be a shared power source for any other electrical device. Still, in other examples, the power source 812 can be a solar energy source. Power source 812 may include a power grid or a generator and may be DC or AC It should be appreciated that the power source 812 may include a combination of any of the above- mentioned sources of power.

The optical fdter 804 can include first and second polarizers, 805 and 807 having a first polarization axis, and a switchable polarizer 806 characterized by a second polarization axis, substantially orthogonal to the first axis. One or both of the outer polarizers 805, 807 may in some cases be static (non-switchable) polarizers. In some embodiments, one or both of outer polarizers 805 and 807 may be switchable or active so long as they have the requisite polarization axis. Polarizers 805, 806, and 807 may be characterized as absorptive polarizers, reflective polarizers, broadband polarizers, or narrow band polarizers, among others.

An absorptive polarizer is a polarizer that can primarily absorb a selected polarization of light more than the other. An absorptive linear polarizer can have two propagation Eigen modes referred to herein as axes. These axes can include an absorptive axis and a transmissive axis, which can be orthogonal to each other. The polarization of the light that is aligned with the absorptive axis can be absorbed more than the perpendicular polarization. In some examples, the polarizer can be designed/configured to be wavelength dependent. In some cases, a broadband polarizer may polarize a broad band of light, e.g., light having a full-width, half-max absorption equal to or greater than 175 nm. In some cases, a narrowband polarizer may polarize a narrow band of light, e.g., light having a full-width, half-max absorption of less than 175 nm.

For example, an absorptive linear polarizer with an axis in the x-direction can mean that the polarizer will substantially absorb the x-direction polarization of light while substantially allowing y-direction polarization to propagate. Consequently, an absorptive linear polarizer with an axis in the y-direction means that the polarizer can substantially absorb y-polarized light and substantially transmit x-polarized light. In the examples depicted in the figures, X and Y can be used for the first and second polarization directions (e.g., Eigen modes) for ease of explanation, but it is to be understood that it may represent any polarization state. For example, the polarization states can be any of the following: circular, elliptical, linear, or the like. In another example, absorptive circular polarizers may be constructed by using a linear polarizer in combination with a quarter wave retarder. Once light is polarized by the polarizer, the quarter wave plate can induce a 1/4 phase retardation that can turn a linear polarization to a circular polarization.

A reflective polarizer is a polarizer that may reflect a selected polarization of light more than the other. For example, a reflective polarizer with a reflective axis in the x-direction can indicate that the reflective polarizer will reflect a circular polarization (for example right-handed) of incident light more than the other y-direction polarization (left-handed). Consequently, a reflective polarizer with a reflective axis in the y-direction means that the reflective polarizer can reflect the x-direction polarization (right) of incident light more than the other y-direction polarization (left). As in the case of absorptive polarizers, X and Y can be used to indicate the first and second polarization states for ease of explanation. It should be appreciated, however, that any polarization states such as circular, elliptical, or linear can be used. In the case of circular light, the x and y refer to the handedness of the polarization rather than fixed directions in space. Therefore x, for example, will denote right circular and y will denote left circular. As in the case of linearly polarized light, the unpolarized light will be considered to be composed of equal amounts of left and right circularly polarized amounts. The reflective polarizer, in that case, will reflect either the right- or the left-circular polarization depending on its configuration and will transmit the other polarization, left or right, respectively.

In some cases, a broadband reflective polarizer may polarize a broad band of light, e.g., having a full-width, half-max reflection of 175 nm or greater. In some cases, a narrowband reflective polarizer may polarize a narrow band of light, e.g., having a full-width, half-max reflection of less than 175 nm.

In some embodiments, a polarizer (absorptive, reflective, or other) may be in the form of a single layer structure, or alternatively, have a multilayer structure.

In an example, light can enter optical device 802 as light 820 and exit as light 830. Light 830 can be attenuated by optical device 802 by a certain percentage. For instance, the optical device 802 can be configured to achieve a dark state which is minimally transmissive, a clear or transparent state which is maximally transmissive, or a tinted state falling between the dark state and the transparent state, by manipulating the voltage applied to the switchable polarizer 806. That is, the transparency of the optical filter 804 may be altered between a more transparent state (which may optionally include a clear state in some cases) and a less transparent state (which may optionally include a dark state in some cases).

FIGS. 9 and 10 illustrates an exemplary optical filter 900. In some embodiments, the optical filter 900 may provide an increased viewing angle or FEA with minimal compromise in desired light attenuation or response time when activated. The optical filter 900 includes a first polarizer 902 and second polarizer 904, where the first and the second polarizers are considered outer polarizers. The optical filter 900 can further include an inner switchable polarizer 910 provided between the first polarizer 902 and the second polarizer 904. The first polarizer 902 and the second polarizer 904 can each have a first polarization axis effectively parallel to one another, and the inner switchable polarizer 910 can have a second polarization axis effectively orthogonal to that of the first polarizer 902 and the second polarizer 904. In some embodiments, the first and the second polarizers 902 and 904 can be static (non-switchable) polarizers. In some other embodiments, one or both of the first and second polarizers 902 and 904 can be switchable polarizers that have or can produce a polarization axis effectively parallel to each other and approximately orthogonal to the inner switchable polarizer in some instances as needed.

In some embodiments, whether switchable or not, a second polarizer 904 may have its physical axis of polarization positioned at an angle that is substantially perpendicular to the polarization axis of the first polarizer 902, provided that there is a waveplate or polarization rotator positioned between the first polarizer and the second polarizer (e.g., between the second polarizer 904 and the inner switchable polarizer 910). The waveplate can rotate received polarized light by 90 degrees to make the second polarizer effectively parallel to the first polarizer.

In some cases, the inner switchable polarizer 910 may include a guest host liquid crystal cell. The inner switchable polarizer 910 may be switched between a more polarizing state and a less polarizing state.

In some embodiments as shown in FIGS. 9 and 10, the first polarizer 902 and the second polarizer 904 can each have a first polarization axis that is transmissive to Y-polarized light, but is generally not transmissive to X-axis polarized light. Inner switchable polarizer 910 is switchable between a less polarizing state and a more polarizing state. The less polarizing state may be generally transmissive to both polarizations of light (X and Y) such that incident light may pass through switchable polarizer 910 without substantial attenuation. The more polarizing state may be generally transmissive to X-polarized light, but generally not transmissive to Y- polarized light.

FIG. 9 illustrates optical filter 900 in a more transparent or clear state. In operation, nonpolarized light 920 having both X and Y polarizations (X,Y) impinges on first polarizer 902 having a first polarization axis that transmits Y-polarized light, but that generally does not transmit X-polarized light. The first polarizer may be absorptive or reflective as previously discussed. Thus, predominantly Y-polarized light reaches the inner switchable filter 910 which is in a less polarizing or transmissive state. In this example, the inner polarizer is a guest-host LC cell having a positive anisotropy dichroic dye guest in a negative dielectric anisotropy LC. In this configuration, a less polarizing state means the liquid crystal and dye of the guest host cell 912 align in a direction approximately perpendicular to the cell substrates 914, referred to as homeotropic alignment. The incident Y-polarized light is thus transmitted through the inner switchable polarizer 910 without substantial attenuation (when in this less polarizing state). The Y-polarized light is then incident on the second polarizer, which is also generally transmissive to the Y-polarized light, to produce transmitted light 930. It should be noted that some X-polarized light may also be present, but in this case, the majority of the light 930 is Y-polarized. In some embodiments, the transmissivity in the more transmissive state can be at least about 10%, alternatively at least about 12%, 15%, 18%, 20%, 25%, 30%, 40% or 50%.

In some applications, it is desirable to achieve a device transmissivity that is higher than 50%. One way this can be achieved is to use switchable polarizers for one or both outer polarizers 902 and 904. For example, if polarizer 902 is a switchable polarizer in its less polarizing (more transparent) state, then both X- and Y-polarizations of incident light 920 will be transmitted through with relatively low attenuation to inner switchable polarizer 910. When inner switchable polarizer 910 is in its less polarizing (more transparent) state it will pass the incident X-and Y-polarized light through to outer switchable polarizer 904 with relatively low attenuation. If outer switchable polarizer 904 is in its less polarizing (more transparent) state, it will pass transmitted light 930 with relatively low attenuation. Thus, when all switchable polarizers 902, 910 and 904 are in their less polarizing state, the optical filter may in some cases have a transmissivity of at least 50%, 60%, 70%, 80%, or 90%.

FIG. 10 illustrates the optical filter 900 in a less transmissive state, e.g., in a dark state. Here, the inner switchable polarizer 910 is in its polarizing state designed to preferentially absorb or reflect Y-polarized light. In this example, the switchable polarizer 910 is in a planar or homogeneous alignment where the liquid crystal and dye are aligned in a direction that is approximately parallel to the substrates 914. As such, a portion of the incident Y-polarized light is blocked (absorbed or reflected). As a result, light emerging from optical filter 910 as transmitted light 930 is substantially attenuated. Note that even in a dark state, there can be some small amount of light transmitted. In some embodiments, e.g., where the guest host cell is chosen to have a broad band absorption, the optical filter 900 can achieve a transmissivity of about 1% corresponding to an OD 2 rating. In some embodiments, the transmissivity can be 0.1% corresponding to an OD 3 rating, 0.01% corresponding to an OD 4 rating, or 0.001% corresponding to an OD 5 rating. In some embodiments, the dark state OD may be at least 1.0, alternatively at least 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0.

The inner switchable polarizer 910 can be switched by a change in an applied voltage. For example, inner switchable polarizer 910 may be in a more transmissive or clear state (less polarizing) when no voltage is applied, and in a less transmissive or dark state (more polarizing) when a voltage is applied. Alternatively, the inner switchable polarizer 910 may be in a less transmissive or dark state (more polarizing) when no voltage is applied, and in a clear state (less polarizing) when a voltage is applied.

In some embodiments, when the optical filter is in its less transparent state, transmitted light 930, to the degree that it is visible, may appear neutral color (almost black or gray). For example, this may be the case when using a switchable polarizer that reflects or absorbs broadband light across the visible spectrum. In some embodiments, transmitted light 930 may have a distinct color. For example, this may be the case when using a switchable polarizer that reflects or absorbs narrow-band light within the visible spectrum.

In some embodiments, the inner switchable polarizer can be a cholesteric LC, known as a CLC. Reflective CLCs are known in the art.

FIG. 11 illustrates an exemplary guest host (GH) liquid crystal (LC) cell 1100. The guest host liquid crystal cell 1100 can include substrates 1102 and 1104, electrode layers 1106 and 1108, and a guest host liquid crystal mixture 1120 disposed between the substrates. The guest host liquid crystal mixture can include a plurality of liquid crystals 1122 and/or dye 1124 which may be a dichroic dye. The transparent electrode layers 1106 and 1108 can be disposed on the substrates 1102 and 1104 such that each electrode is interposed between its respective substrate and the guest host liquid crystal mixture. Substrates 1102 and 1104 may be flexible or rigid and may be made from a plastic or a glass material. Some non-limiting examples of substrates include polycarbonate and Corning Willow Glass. Substrates are generally transparent, but in some cases, may have a hue or coatings that affect the optical transmission.

In an exemplary embodiment, the liquid crystal cell 1100 may include a first alignment layer 1110 disposed on the electrode layer 1106 and a second alignment layer 1112 disposed on the electrode layer 1108. In an example, spacers (not shown) can be used to maintain separation between various layers of the guest host liquid crystal cell 1100. The electrode layers 1106 and 1108 can be indium tin oxide (ITO) or other suitable transparent conductive materials. In another embodiment, the liquid crystal cell 1100 can include first alignment layer 1110 and not second alignment layer 1112. Yet, in another embodiment, the liquid crystal cell 1100 can include second alignment later 1112 and not first alignment layer 1110.

In an embodiment, the liquid crystal 1122 can have a positive anisotropy or, in alternative embodiments, can have a negative anisotropy. Similarly, the dichroic dye 1124 can be chosen to have a positive, or in alternative embodiments, a negative dielectric anisotropy. In an example, the liquid crystal cell gap can be between 3-50 microns. In other examples, the cell gap can be around 4-12 microns with a spatial variation of +/- 10%. The contrast ratio of the filter can be greater than 7:1, 10: 1, 20:1, 50:1, 100:1, 1000:1, or 10,000:1.

In certain embodiments, the liquid crystal cell 1100 can be flat. Yet, in other embodiments, the liquid crystal cell 1100 can have curvature in at least one dimension and can have a diopter of greater than zero. In other embodiments, the liquid crystal cell 1100 can have curvature in multiple directions with curvature along one direction of at least 0.1, 0.5, 1, 2, 3, 4, 5, or 6, for example.

In some examples, the guest host mixture 1120 includes a host liquid crystal 1122 and at least one guest dye or dye mixture 1124. In some cases, a switchable polarizer using such a guest host mixture can have a switchable polarization that can be altered between a more transmissive or less polarizing state, where both polarization directions of light are transmitted, and a more polarizing state, where one polarization direction of light is reflected or absorbed more than the other. For example, the inner switchable polarizer 910 can alternate between the more transmissive state and the less transmissive state by changing the voltage applied to the electrodes 1106 and 1108. As mentioned, the electric field can cause the liquid crystal 1122 and dye 1124 molecules to orient themselves in a direction generally perpendicular to the orientation of the substrates 1102 and 1104 forming a homeotropic orientation or parallel to the substrates 1102 and 1104 forming a planar orientation.

By way of example, a homeotropic state while voltage at the liquid crystal cell 1100 is zero (e g., V=0) can be achieved when there is -ve LC at the liquid crystal cell (e g., clear at rest or at V=0). A planar state at V=0 can be achieved when there is +ve LC at the liquid crystal cell (e.g., dark at rest or V=0). When using a positive dye, the guest host mixture can absorb light when in a planar orientation and can transmit light when in a homeotropic orientation. A polarizer state can be described as when liquid crystal 1122 and dye 1124 align in a generally parallel orientation to the substrates 1102 and 1104. A transmissive state can be described as when liquid crystal 1122 and dye 1124 align in a generally perpendicular orientation to the substrates 1102 and 1104 allowing transmission of light. The change in applied voltage can be done automatically, manually, or remotely through a control board (not shown). The voltage can also be controlled by a controller, such as controller 810, for example, when a certain level of light at one or more sensors 808 is detected. In some examples, the guest-host mixture can have an order parameter that is greater than 0.6, or 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79 or 0.8, or any number in between 0.6 and 0.8.

In an embodiment, the optical filter 900 can be configured for wide band absorption. In some applications, the entire visible light spectrum or a wide band may need to be absorbed. Wide band absorption can be defined as a spectral absorption band that is greater than 175 nm in one embodiment, and greater than 180 nm, 185 nm, 1 0 nm, 195 nm or 200 nm in other embodiments, where the entire spectral absorption band is contained within the range of visible wavelengths, typically assumed to be 400 nm-700 nm.

In an example, wide band absorption can be accomplished by using a wide band dye. The absorption band width, or the width of the absorption curve, may be calculated by what is known in the art as an “Aggregate Full Width at Half Maximum” (A-FWHM). In this example, a wide band dye can be considered to be any dye or mixture of dyes that results in a spectral absorption band characterized by an A-FWHM that is equal to or greater than 175 nm in one embodiment, and greater than 180 nm, 185 nm, 190 nm, 195 nm or 200 nm in other embodiments, where the entire spectral absorption band is contained within the range of visible wavelengths. A-FWHM can be understood as follows: full width at half maximum (FWHM) is a measurement that characterizes the width of the absorption curve of the dye. It is defined as the distance between the cut-off points on an absorption curve that occur where the absorption is one-half of the maximum absorption. Thus, regardless of the shape of the absorption curve, the FWHM is the width of the curve between the two cut-off points where the absorption is one-half of the maximum absorption.

In some embodiments, a polarizer or optical filter can be configured for narrow band absorption. In certain applications, it is desirable to have a switchable inner polarizer (e g. a guest host cell) that can block certain wavelengths, or wavelength regions, of light according to what is needed. For example, if laser light of a particular color or wavelength is directed towards the optical filter, it may be desirable for the optical filter to adapt quickly and block the selected region of the spectrum that corresponds to the laser. This can reduce or eliminate damage caused by the laser light. In other embodiments, the optical filter can be adapted to have two or more guest host cells, where each can be designed to block a particular region of the light spectrum. In these embodiments, the device can be capable of switching between the two guest host cells as the incident laser or other bright light source changes color or wavelength.

FIG. 12 illustrates an exemplary embodiment of an optical filter 1200. The optical filter 1200 can include a first polarizer 1202 and a second polarizer 1204, which may also be referred to as the outer polarizers. The optical filter 1200 can further include a first switchable polarizer 1210, a second switchable polarizer 1212, and a third switchable polarizer 1214, referred to as the switchable polarizers or inner switchable polarizers.

In this embodiment, first, second, and third switchable polarizers 1210, 1212, and 1214 can include guest host liquid crystal cells. Each guest host liquid crystal cell can be configured to filter a particular region of the light spectrum. The first and second polarizers 1202 and 1204, respectively, can have an effectively parallel polarization axis thereby absorbing or reflecting the same polarization axis of light. In some embodiments, the optical filter 1200 can also include a band pass filter (not shown) to filter out desired wavelengths such as infrared and ultraviolet light. In other embodiments, the optical filter 1200 can be curved in shape (not shown).

In one example, when guest host cells of the switchable polarizers are in their clear or transparent state, the liquid crystal and dye can be in a homeotropic orientation to allow light 1220 to pass through un-polarized. In a dark state, the liquid crystal and dye can be in a planar orientation, and can absorb or reflect a specified region of light in the spectrum. The operation may in some cases be generally analogous to that described with respect to optical filter 900. First and second outer polarizers may each be characterized by a first polarization axis, i.e., they are effectively parallel to each other in this regard. The inner switchable polarizers (in the more polarizing state) are characterized by a second polarization axis that is effectively orthogonal to the first polarization axis.

For example, the first polarizer 1202 and the second polarizer 1204 can each have a first polarization axis that is transmissive to Y-polarized light, but is generally not transmissive to X- axis polarized light. Inner switchable polarizers 1210, 1212, 1214 are individually switchable between a less polarizing state and a more polarizing state. A less polarizing state may be generally transmissive to both polarizations of light (X and Y) such that incident light may pass through a switchable polarizer without substantial attenuation. A more polarizing state may be generally transmissive to X-polarized light, but generally not transmissive to Y-polarized light. Although first and second polarizers 1202 and 1204 are discussed in this example as static polarizers, in some embodiments one or both can also be switchable polarizers.

In operation, non-polarized white light having red (R), green (G), and blue (B) color components and both X and Y polarizations (X,Y) may impinge the outer first polarizer 1202 which allows only Y-polarized red, green and blue light to pass (Y/R,G,B). In FIG. 12, the first switchable red polarizer 1210 is in its more polarizing state so that it absorbs or reflects the incident red light, but allows the green and blue light through. The other switchable polarizers are each in their less polarizing state and also allow the Y-polarized green and blue light through. Outer second polarizer 1204 similarly allows Y-polarized green and blue light to be transmitted so the optical filter 1200 in this configuration transmits light 1230 that is cyan (blue-green) in color, with a substantial amount of the red light from light 1220 now removed. A representative absorption spectrum of the optical filter 1200 in this configuration is shown in FIG. 13.

In some other embodiments, instead of the first switchable polarizer 1210 being in its more polarizing state, just the green second switchable polarizer 1212 may be, or alternatively just the blue third switchable polarizer 1214 may be in a polarizing state. Representative absorption spectra for these states are shown in FIG. 14 where spectrum R is that for the red first switchable polarizer in its more polarizing state, spectrum G is that for the green second switchable polarizer in its more polarizing state, and spectrum B is that for the blue third switchable polarizer in its more polarizing state. Any combination of red, green, and blue switchable filters may be in their more polarizing state. Further, as indicated elsewhere the intensity of light attenuation for each color can in some cases be controlled across a range rather than simply switched between two states.

FIG. 15 illustrates another non-limiting example of an optical filter according to some embodiments Optical filter 1500 may include a first cholesteric liquid crystal (CLC) polarizer 1503, a second CLC polarizer 1505, an absorptive switchable polarizer 1510 disposed between the first and second CLC polarizers, a first quarter wave plate 1542 disposed between the first CLC polarizer and the absorptive switchable polarizer, and a second quarter wave plate 1544 disposed between the absorptive switchable polarizer and the second CLC polarizer. In some cases, the switchable polarizer 1510 and quarter wave plates 1542, 1544 may collectively be considered a switchable multilayer absorptive circular polarizer 1511. In other non-limiting examples, the switchable polarizer can also alter the polarization state of the transmitted light in one of its states. This may allow for a normally dark or a normally clear device operation.

In some embodiments, the first and second CLC polarizers may be configured to reflect right-handed circularly polarized light. When the non-polarized light 1520 impinges on the first CLC polarizer 1503, right-handed circularly polarized light is reflected, and left-handed circularly polarized light 1521 is transmitted through to the first quarter wave plate 1542. The first quarter wave plate receives and acts on the left-handed circularly polarized light to produce substantially linearly polarized light 1523 having a first polarization axis.

In some cases, the LC cell may have a guest host configuration wherein the host may have a negative anisotropy. In its less polarizing state, switchable polarizer 1510 allows incident polarized light 1523 to pass substantially unaltered as polarized light 1525 (generally still polarized along a first polarization axis). In its more polarizing state, switchable polarizer may absorb or reflect a substantial amount of incident polarized light 1523 and allow relatively little light through as 1525.

Polarized light 1525 (if any) is received by the second quarter wave plate 1544 which substantially transforms the polarized light having a first polarization axis to left-handed circularly polarized light 1527. The second CLC polarizer 1505 receives and transmits the lefthanded circularly polarized light to produce transmitted light 1530. Note that right-handed circularly polarized light that may have leaked through or produced by the second quarter wave plate will be reflected by the second CLC polarizer. Although the first and second CLC polarizers were discussed as reflecting right-handed circularly polarized light, they both may instead be selected to reflect left-handed circularly polarized light.

It should be noted that switchable polarizer 1510 may in some embodiments be a reflective polarizer such as a CLC, rather than an absorptive, guest-host LC type of polarizer as described above. In such embodiments using a switchable reflective polarizer, the quarter wave plates may not be needed.

Still further embodiments herein include the following enumerated embodiments.

1. An optical filter comprising: first and second outer polarizers each configured to effectively polarize light in a first polarization axis; and a first inner switchable polarizer disposed between the first outer polarizer and the second outer polarizer, wherein the first inner switchable polarizer is configured to alternate between a more polarizing state and a less polarizing state upon a change of an applied voltage, the first inner switchable polarizer configured to polarize light in a second polarization axis effectively orthogonal to the first polarization axis when the first inner switchable polarizer is in the more polarizing state, and wherein the change in applied voltage alters a transparency of the optical filter from a more transparent state to a less transparent state.

2. The optical filter of embodiment 1, wherein the first inner switchable polarizer is an absorptive polarizer, a reflective polarizer, or a combination thereof.

3. The optical filter of embodiment 1 or 2, wherein the first inner switchable polarizer comprises a liquid crystal cell configured to absorb light comprising a mixture of a liquid crystal host and one or more guest dyes provided between a pair of substrates.

4. The optical filter of embodiment 3, wherein the one or more guest dyes includes an absorption band of 150 nm or greater.

5. The optical filter according to any of embodiments 3 - 4, wherein at least one of the pair of substrates comprises a flexible material.

6. The optical filter according to any of embodiments 1 - 5, wherein the less transparent state has an optical density (OD) of at least 2 for normally incident light. 7. The optical filter according to any of embodiments 1 - 6, further characterized by an acceptance aperture angle of at least 30 degrees from normally incident light, wherein an optical density rating at the acceptance aperture angle is decreased by no more than one as compared to an optical density rating of normally incident light.

8. The optical filter according to any of embodiments 1 - 7, wherein the optical filter is chosen from the following options:

(a) configured to be in the more transparent state when the voltage is zero; or

(b) configured to be in the less transparent state when the voltage is zero.

9. The optical filter according to any of embodiments 1 - 8, wherein the optical filter is flexible.

10. The optical filter according to any of embodiments 1 - 9, wherein the optical filter is curved.

11. The optical filter according to any of embodiments 1 - 10, wherein the first inner switchable polarizer is configured to absorb or reflect a narrow-band region of visible light or a broad-band region of visible light.

12. The optical filter according to any of embodiments 1 - 11, further comprising an infrared filter, an ultra-violet filter, or both.

13. The optical filter according to any of embodiments 1 - 14, further comprising a second inner switchable polarizer disposed between the first inner switchable polarizer and the second outer polarizer, wherein: i) the second inner switchable polarizer is configured to alternate between a more polarizing state and a less polarizing state upon a change of an applied voltage, the second inner switchable polarizer configured to polarize light in the second polarization axis when the second inner switchable polarizer is in its more polarizing state; ii) the first inner switchable polarizer is configured in its more polarized state to attenuate a first range of wavelengths of visible light; and iii) the second inner switchable polarizer is configured in its more polarized state to attenuate a second range of wavelengths of visible light.

14. The optical filter of embodiment 13, wherein the first range is different from the second range. 15. The optical filter of embodiment 13 or 14, wherein one or both of the first and second inner polarizers are configured to absorb or reflect a narrow-band region of visible light.

16. The optical filter according to any of embodiments 13 - 15, wherein the first range or the second range_of wavelengths comprise a range within a red light spectrum, a green light spectrum, or a blue light spectrum

17. The optical filter of embodiment 13, wherein one or both of the first and second inner polarizers are configured to absorb or reflect a broad-band region of visible light.

18. The optical filter according to any of embodiments 13 - 17, further comprising a third inner switchable polarizer disposed between the second inner switchable polarizer and the second outer polarizer, wherein: i) the third inner switchable polarizer is configured to alternate between a more polarizing state and a less polarizing state upon a change of an applied voltage, the third inner switchable polarizer configured to polarize light in the second polarization axis when the third inner switchable polarizer is in its more polarizing state; and ii) the third inner switchable polarizer is configured in its more polarized state to attenuate a third range of wavelengths of visible light.

19. The optical filter of embodiment 18, wherein the third range is different than the first or second ranges

20. The optical filter of embodiment 19, wherein the third range of wavelengths comprises a range within a red light spectrum, a green light spectrum, or a blue light spectrum.

21. The optical filter of embodiment 18 or 19, wherein a color of transmitted light may be changed by selectively altering the polarization states of the inner first switchable polarizer, the inner second switchable polarizer, the inner third switchable polarizer, or a combination thereof.

22. The optical filter according to any of embodiments 1 - 21, wherein one or both of the first and second outer polarizers are configured as switchable polarizers.

23. The optical filter according to embodiment 22, wherein the more transparent state comprises greater than 50% transmissivity when all switchable polarizers are in the less polarizing state.

24. The optical filter according to any of embodiments 1 - 23, further comprising at least one waveplate positioned between an outer polarizer and an inner switchable polarizer. 25. The optical filter of embodiment 24, wherein the waveplate is a quarter waveplate.

26. The optical filter of embodiment 24, wherein the waveplate is a half waveplate.

27. An optical filter comprising: first and second outer cholesteric liquid crystal (CLC) polarizers configured to effectively polarize light in a first polarization axis; and a switchable polarizer disposed between the first and second outer CLC polarizers, wherein the inner switchable polarizer is configured to alternate between a more polarizing state and a less polarizing state upon a change of applied voltage, the inner switchable polarizer configured to polarize light in a second polarization axis effectively orthogonal to the first polarization axis when the inner switchable polarizer is in the more polarizing state, and wherein the change in applied voltage alters a transparency of the optical filter from a more transparent state to a less transparent state.

28. The optical filter of embodiment 27, wherein the inner switchable polarizer comprises a reflective polarizer comprising a CLC material.

29. The optical filter of embodiment 27, further comprising: a first quarter waveplate disposed between the first outer CLC polarizer and the inner switchable polarizer; and a second quarter waveplate disposed between the inner switchable polarizer and the second outer CLC polarizer, wherein the inner switchable polarizer comprises an absorptive polarizer comprising an LC guest-host material.

30. An optical device comprising an optical filter according to any of embodiments 1 - 29, the optical device comprising a near-eye display, goggles, eyewear, a visor, a welding helmet, a window, a see-through display, or a space suit visor.

In the specification and claims, reference will be made to a number of terms that have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Approximating language, as used herein throughout the specification and claims, may be applied to modify a quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Moreover, unless specifically stated otherwise, a use of the terms “first,” “second,” etc., do not denote an order or importance, but rather the terms “first,” “second,” etc., are used to distinguish one element from another.

As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur - this distinction is captured by the terms “may” and “may be.”

This written description uses examples to disclose the subject matter, including the best mode, and also to enable one of ordinary skill in the art to practice the invention, including making and using a devices or systems and performing incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differentiate from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.